Remove undefined from type

Question

I use typeof to infer the return type of a function, but since I cannot call the actual function I use a trick using the ternary operator to infer the type, however this leaves me with a union type that includes undefined:

function foo() {
  return { bar: 1 };
}

const fooInstance = true ? undefined : foo(); // foo() is never actually called
type FooOrUndefined = typeof fooInstance;     // {bar: number} | undefined 
type Foo = ???;                               // Should be { bar: number }

Is there any way to get rid of undefined from FooOrUndefined?

Answer 1

You will want to use NonNullable:

type Foo = NonNullable<FooOrUndefined> // { bar: number; }

Sample

Answer 2

If you just want to remove undefined but keep null, you can do a small util:

type NoUndefined<T> = T extends undefined ? never : T;

type Foo = number | string | null | undefined;

type Foo2 = NoUndefined<Foo> // number | string | null